# Assumptions in Newton's Second Law

With my understanding of Newton's second -- if I am going to use it to solve a problem, I would need to write ALL velocities in the system with respect to the lab frame (inertial rest frame) -- correct?

Thanks,
Chris

A.T.
With my understanding of Newton's second -- if I am going to use it to solve a problem, I would need to write ALL velocities in the system with respect to the lab frame (inertial rest frame) -- correct?
Velocity doesn't even appear in Newton's second law.

It does in the rocket problem. ##\dot{p}=\dot{m}v+m\dot{v}=F_{thrust}##. Also, if you are calculating the the change in momentum over some time interval ##F=m\frac{\Delta v}{\Delta t}##.

Chris

A.T.